As a process for forming interconnects in a semiconductor substrate, a so-called “damascene process”, which comprises embedding a metal (electric conductor) into interconnect trenches and contact holes, is coming into practical use. According to this process, aluminum, or more recently a metal such as silver or copper, is embedded into interconnect trenches and contact holes previously formed in an interlevel dielectric layer. Thereafter, an extra metal is removed by performing chemical mechanical polishing (CMP) so as to flatten a surface of the substrate.
In a case of interconnects formed by such a process, for example, copper interconnects formed by using copper as an interconnect material, embedded interconnects of copper have exposed surfaces after the flattening processing. In order to prevent thermal diffusion of such interconnects (copper), or to prevent oxidation of such interconnects (copper) e.g. during forming thereon an insulating film (oxide film) under an oxidizing atmosphere to produce a semiconductor device having a multi-level interconnect structure, it is now under study to selectively cover the exposed surfaces of interconnects with an interconnects-protective layer (cap material) composed of a Co alloy, a Ni alloy or the like so as to prevent thermal diffusion and oxidation of the interconnects. Such an interconnects-protective layer of a Co alloy, a Ni alloy or the like can be produced e.g. by performing electroless plating.
As shown in FIG. 1, for example, fine recesses (interconnect trenches) 4 for interconnects are formed in an insulating film (interlevel dielectric layer) 2 of SiO2 or the like which has been deposited on a surface of a substrate W such as a semiconductor wafer. A barrier layer 6 of TaN or the like is formed on a surface of the insulating film 2, and then copper plating, for example, is carried out onto the surface of the substrate W to fill the recesses 4 with copper and deposit copper film on the surface of the substrate W (damascene process). Thereafter, CMP (chemical mechanical polishing) is carried out onto the surface of the substrate W so as to flatten the surface of the substrate, thereby forming interconnects 8 composed of copper in the insulating film 2. Thereafter, an interconnects-protective layer (cap material) 9 composed of a CoWP alloy film is formed e.g. by electroless plating selectively on the surfaces of interconnects (copper) 8 to protect interconnects 8 (cap plating process).
A common electroless plating method for the selective formation of the interconnects-protective layer (cap material) 9 of the CoWP alloy film on the surfaces of interconnects 8 generally involves the following process steps: First, the substrate W such as a semiconductor wafer, which has undergone the CMP treatment, is immersed in an acid solution (first processing liquid) e.g. of 0.5 M H2SO4 at the solution temperature of e.g. 25° C. for e.g. one minute to remove CMP residues, such as copper, remaining on a surface of an insulating film 2. The surface of the substrate W is then cleaned with a cleaning liquid (second processing liquid) such as ultrapure water (pre-cleaning process).
Next, the substrate W is immersed in a mixed solution (first processing liquid), e.g. of 0.005 g/L PdCl2 and 0.2 ml/L HCl, at the solution temperature of e.g. 25° C. for e.g. one minute to adhere Pd as a catalyst to the surfaces of interconnects 8, thereby activating the exposed surfaces of interconnects 8. The surface of the substrate W is then cleaned with a cleaning liquid (second treatment liquid) such as ultrapure water (first pre-processing process).
Next, the substrate W is immersed in a solution (first processing liquid) containing e.g. 20 g/L of Na3C6H5O7.2H2O (sodium citrate) at the solution temperature of e.g. 25° C., thereby carrying out neutralization treatment of the surfaces of interconnects 8. The surface of the substrate W is then cleaned with ultrapure water (second processing liquid) (second pre-processing process).
Next, the substrate W is immersed in a CoWP plating solution at the solution temperature of e.g. 80° C. for e.g. 120 seconds, thereby carrying out selective electroless plating (electroless CoWP cap plating) onto the activated surfaces of interconnects 8. Thereafter, the surface of the substrate W is cleaned with a cleaning liquid such as ultrapure water (plating process). The interconnects-protective layer 9 composed of a CoWP alloy film is thus formed selectively on the surfaces of interconnects 8 to protect interconnects 8.
A dip processing method, which involves immersing a substrate in a processing liquid to bring the surface (surface to be plated) of the substrate into contact with the processing liquid, has conventionally been employed for carrying out stable and uniform plating (e.g. electroless plating) of the substrate or stable and uniform pre-plating processing or cleaning of the substrate. A substrate processing apparatus adapted for the dip processing method is generally provided with a substrate holding apparatus for holding a substrate while sealing a peripheral portion of the front surface of the substrate, so that when the substrate, held by the substrate holding apparatus, is immersed in a processing liquid for processing of the substrate, the processing liquid is prevented from intruding into the peripheral portion of the front surface and also into the back surface of the substrate.
A substrate holding apparatus which employs a so-called vacuum attraction method has been developed. Such a substrate holding apparatus includes a ring-shaped attraction seal comprised of an elastic body such as a rubber, and presses the attraction seal against a substrate so as to bring the end surface of the attraction seal into tight contact with a peripheral portion of the back surface of the substrate over the entire circumference, and attracts and holds the substrate while sealing the peripheral portion of the back surface of the substrate in a ring with the attraction seal by internally vacuuming the attraction seal.
A substrate processing apparatus (plating apparatus) for carrying out various pre-processing steps, cleaning steps, etc. is requested to carry out various processings with good quality as a matter of course, and in addition, to be made compact or downsized and to increase the throughput.
It is, however, difficult to meet these requests especially with a substrate processing apparatus that employs the dip processing method. In particular, in order to ensure a sufficient immersion depth of a substrate, held by a substrate holding apparatus, in a processing liquid to improve the quality of processing, it is necessary, for example, to provide a weir or the like for closing off the flow of processing liquid on the substrate-fixing side, for example, on the side of a substrate holder for supporting the substrate, of the substrate holding apparatus so as to prevent the processing liquid from intruding into the back surface side of the substrate. This leads to an increased size not only of the substrate holding apparatus itself but also of a robot hand or the like for carrying a substrate in the substrate holding apparatus, and thus is incompatible with the request for a smaller-sized apparatus.
With respect to a substrate holding apparatus that employ the above-described vacuum attraction method, because of the dimensional tolerance of each member constituting the apparatus, variations between substrates, a biased pressure acting on an attraction seal comprised of an elastic body, etc., the end surface of the attraction seal cannot securely be contacted tightly, without an empty space, with a peripheral portion of the back surface of a substrate, which is held horizontally, over the entire circumference prior to vacuuming of the attraction seal. This could cause air leakage when attracting and holding a substrate by the substrate holding apparatus whereby the substrate cannot be attracted and held.